Measuring contaminant transport rates between water and sediments using limnocorrals

Abstract

Radiotracers injected as soluble salts into 1.2 m 3 limnocorrals in a shallow dystrophic lake were transported rapidly (2 to 12% d -) from the water to the bottom sediments. Removal rates of most contaminants declined after 14 days. Tracers were removed from the water much more rapidly than stable element analogs present naturally. After 3 weeks Am, Co, Ra, Hg, Sn and Fe activities on the enclosure walls were greater than 15% of the activity in the water. However, activity on the walls was a small fraction (< 6%) of the total amount of tracer injected. Particle fluxes inside the corrals were lower and much less variable than those measured outside. This difference appears to result from greater resuspension of bottom sediments in the lake than in the enclosures. Both particle-bound and soluble tracers were measured in cores of bottom sediments. Tracer sorption onto particles, diffusion into the bottom sediments, and uptake on the plastic enclosure walls were rapid and reversible. Tracer kinetics were very reproducible in replicate enclosures, providing a simple, experimental system in which limnological conditions can be manipulated. Loss rates and distributions of stable isotopes and radioisotopes can be used to develop and test a general model of element transport applicable to both short and long term analyses.

The Transfer of Iodine and Technetium from Surface Waters to Sediments

“…These rates are slightly lower than the loss rates of all the other radiotracers added (3 to 12% d" 1 )[15], except 22 Na (loss rate 1-1.8% d -1 )[13] and "Tc. The rates of radioiodine transport are very similar to those measured in two other whole-lake experiments.…”

“…The enclosures were open to both the atmosphere and the underlying organic (gyttja) sediments. Details of this methodology have been reported by Chant and Cornett [13].…”

“…Methodological tests for leakage, wall uptake and disturbance, performed throughout the experiment [13], indicated that these effects were negligible. Less than 1.5% of the 131 I activity was lost to the enclosure walls during the experiments.…”

“…The initial radionuclide concentrations ranged from 2000 to 4000 Bq Γ 1 . Over the following 21 days integrated water column samples were collected to measure "Tc and 13 4 in the water, suspended particles were trapped by 0.4 μηι Nucleopore polycarbonate filters, and settling particles were collected in cylindrical sediment traps.…”

“…Therefore, changes in "Tc, 125 I, and 131 I concentration (decay corrected to the start of the experiments), can be interpreted as transport between water and bottom sediments. 13 'I was detected by gamma spectrometry (HPGe) using the 365 keV gamma ray. Some purification was required before it was possible to measure the 125 I by coincidence counting of the X-ray and 35 keV gamma ray.…”